The present invention relates to electronics generally, and more specifically to microwave filters.
Surface-mount millimeter-wave (mm-wave) and radio frequency (RF) components are highly desirable in terms of reducing the manufacturing costs, increasing the repeatability and increasing the performance. Such components are widely used in today""s modern telecommunications systems such as cellular phones and radios. However, they are still not available in high volumes for very high frequency applications such as Local Multipoint Distribution System (LMDS) and Autonomous Cruise Control (ACC) radar for automobiles.
Electrical filters are the basic building blocks that can be found in almost every type of electrical circuitry. Designing of electrical filters has a very well established theory given in the literature. Although there are many ways of implementing the electrical filters, printed microstrip line filters are frequently used in modern RF and millimeter-wave circuits and systems. This is because they are easy to implement, cost-effective, and reproducible through photolithographic techniques. However, making the millimeter-wave printed circuit filters suitable for high-volume manufacturing is a challenge due to the high printing resolution requirements of the filters. In other words, line widths, line lengths, and gaps between the lines of the printed filter should be kept below certain tolerance levels to ensure good performance. The tolerance requirements become more stringent as the frequency increases as one may easily expect. For instance, in order to design a band-pass filter at 77 GHz on a 5-mil thick RT/Duroid 5880 board with relative dielectric constant 2.2 may require the line width and spacing tolerances less than +/xe2x88x920.0025 centimeters (1 mil). This tolerance requirement may not be feasible for low-cost high-volume manufacturing under current technology although it may be supported for prototype development. If the tolerance requirements on the printed filter are not achieved, the response of the filter deviates from the ideal response that affects the yield of the circuitry. Besides, the microstrip line filters have conductor loss in high frequencies.
In most cases, the high-resolution requirement is needed only at certain sections of the circuitry where the filters are implemented. Therefore, one can make the filter sections as separate blocks and then integrate with the main circuit board using wire-bonds. As a result, the main circuit board can be manufactured with relatively low resolution, which reduces the price of manufacturing, while the filters are being manufactured with high accuracy to comply with the specifications. However, even though this solution may address the accuracy problem, it does not provide a solution to the high conductor losses associated with the microstrip lines. Besides, this approach may complicate the assembly process.
Surface mountable transverse electromagnetic mode (TEM) filters are known in the literature. For instance, U.S. Pat. Nos. 6,060,967, 5,162,760 and 6,064,283 describe examples of surface mountable ceramic filters. In those patents, the filters are constructed in dielectric blocks using appropriate cavities or resonator circuits. The main disadvantage of those structures is that they are complicated and expensive to build because they are not suitable for manufacturing with a monolithic microwave integrated circuit (MMIC) process.
Rectangular waveguides in dielectric substrates are addressed in U.S. Pat. Nos. 6,057,747 and 6,064,350. Those patents employed closely spaced circular vias to form the walls of the waveguide structures, which is disadvantageous at high frequencies due to increased parasitic radiation. However, they did not demonstrate making electrical filters using such structures.
Hence, there is a desire to develop surface-mount millimeter-wave filters in the high frequency range.
The present invention is a filter comprising a dielectric substrate having a major surface including first and second microstrips at first and second ends of the major surface, respectively. First and second microstrip-to-waveguide mode converters are provided on the major surface. The first and second mode converters are connected to the first and second microstrips, respectively. A waveguide is integrally formed from a portion of the major surface between the first and second mode converters. A plurality of irises project from the major surface.